This invention relates generally to power sources used in welding and, more particularly, to welding power sources that have a pre-regulator.
Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage, alternating current, (VAC) signal and provide a high current welding output signal. Around the world, utility power sources (sinusoidal line voltages) may be 200/208 V, 230/240 V, 380/415 V, 460/480 V, 500 V and 575 V. These sources may be either single-phase or three-phase and either 50 or 60 Hz. Welding power sources receive such inputs and produce an approximately 10-75 volt. DC or AC high current welding output.
There are many types of welding power sources that provide power suitable for welding, including inverter-based welding power sources. As used herein, an inverter-type power supply includes at least one stage where DC power is inverted into ac power. There are several well known inverter type power sources that are suitable for welding. These include boost power sources, buck power sources, and boost-buck power sources.
Traditionally, welding power sources were designed for a specific power input. In other words, the power source cannot provide essentially the same output over the various input voltages. More recently, welding power sources have been designed to receive any voltage over a range of voltages, without requiring relinking of the power supply. One prior art welding power supply that can accept a range of input voltages is described in U.S. Pat. No. 5,601,741, issued Feb. 11, 1997 to Thommes, and owned by the assignee of the present invention, and is hereby incorporated by reference.
Many prior art welding power supplies include several stages to process the input power into welding power. Typical stages include an input circuit, a pre-regulator, an invertor and an output circuit that includes an inductor. The input circuit receives the line power, rectifies it, and transmits that power to the pre-regulator. The pre-regulator produces a dc bus suitable for conversion. The dc bus is provided to the invertor of one type or another, which provides the welding output. The output inductor helps provide a stable arc.
The pre-regulator stage typically includes switches used to control the power. The losses in switches can be significant in a welding power supply, particularly when they are hard switched. The power loss in a switch at any time is the voltage across the switch multiplied by the current through the switch. Hard switching turn-on losses occur when a switch turns on, with a resulting increase in current through the switch, and it takes a finite time for the voltage across the switch to drop to zero. Soft switching attempts to avoid turn-on losses by providing an auxiliary or snubber circuit with an inductor in series with the switch that limits the current until the transition to on has been completed, and the voltage across the switch is zero. This is referred to as zero-current transition (ZCT) switching.
Similarly, hard switching turn-off losses also occur when a switch turns off, with a resultant rise in voltage across the switch, and it takes a finite time for the current through the switch to drop to zero. Soft switching attempts to avoid turn-off losses by providing an auxiliary or snubber circuit with a capacitor across the switch that limits the voltage across the switch until the transition to off has been completed, and the current through the switch is zero. This is referred to as zero-voltage transition (ZVT) switching.
There are numerous attempts in the prior art to provide soft-switching power converters or invertors. However, these attempts often either transfer the losses to other switches (or diodes) and/or require expensive additional components such as auxiliary switches and their control circuits. Thus, an effective and economical way of recovering (or avoiding) switching losses in power converters or inverters is desirable. Examples of various attempts at soft switching are described below.
U.S. Pat. No. 5,477,131, issued Dec. 19, 1995 to Gegner discloses a ZVT type commutation. However, a controlled auxiliary switch and a coupled inductor are needed to implement the ZVT. Also, the primary current is discontinuous.
Some prior art designs require discontinuous conduction mode for diode recovery. One such design is found in U.S. Pat. No. 5,414,613. This is undesirable because of excessive high frequency ripple in the power lines.
Gegner also disclosed a ZVS converter that operated in a multi-resonant mode in U.S. Pat. No. 5,343,140. This design produced relatively high and undesirable RMS current and RMS voltage.
Another multi-resonant converter is disclosed in U.S. Pat. No. 4,857,822, issued to Tabisz. This design causes undesirable high voltage stress during ZVS events and undesirable high current stress during ZCS events.
U.S. Pat. No. 5,307,005 also requires an auxiliary switch. Losses occur when the auxiliary switch is turned off. This merely shifts switching losses, rather than eliminating them. Other designs that xe2x80x9cshiftxe2x80x9d losses are shown in U.S. Pat. Nos. 5,418,704 and 5,598,318.
A circuit that requires an auxiliary controlled switch but does not xe2x80x9cshiftxe2x80x9d losses to the auxiliary switch is shown in U.S. Pat. No. 5,313,382. This is an improvement over the prior art that shifted losses, but still requires an expensive controlled switch.
Another design that avoided xe2x80x9closs shiftingxe2x80x9d is shown in U.S. Pat. No. 5,636,144. However, that design requires a voltage clamp for recovery spikes, and 3 separate inductors. Also, the voltages on the inductors is not well controlled.
A zero-current, resonant boost converter is disclosed in U.S. Pat. No. 5,321,348. However, this design requires relatively complex magnetics and high RMS current in the switches and magnitudes. Also, a high reverse voltage is needed for the boost diodes.
When it is not practical or cost effective to use a true ZCT and ZVT circuit, an approximation may be used. For example, slow voltage/current transitions (SVT and SCT) as used herein, describe transitions where the voltage or current rise is slowed (rather than held to zero), while the switch turns off or on.
A typical prior art welding power supply 100 with a pre-regulator 104 and an output convertor or inverter 105 is shown in FIG. 1. An input line voltage 101 is provided to a rectifier 102 (typically comprised of a diode bridge and at least one capacitor). Pre-regulator 104 is a hard-switched boost converter which includes a switch 106 and an inductor 107. A diode 108 allows a capacitor 109 to charge up by current flowing in inductor 107 when the switch 106 is turned off. The current waveform in inductor 107 is a rectified sinusoid with high frequency modulation (ripple).
The amount of ripple may be reduced by increasing the frequency at which switch 106 is switched. However, as the frequency at which a prior art hard switched boost converter is switched is increased to reduce ripple, the switching losses can become intolerable.
Another drawback of some prior art power supplies is a poor power factor. Generally, a greater power factor allows a greater power output for a given current input. Also, it is generally necessary to have more power output to weld with stick electrodes having greater diameters. Thus, a power factor correction circuit will allow a given welding power supply to be used with greater diameter sticks for a given line power. A prior art inverter that provided a good power factor is disclosed in U.S. Pat. Nos. 5,563,777. Many prior art convertors with power factor correction suffer from high switching losses. Examples of such prior art designs are found in U.S. Pat. Nos. 5,673,184; 5,615,101; and 5,654,880.
One type of known output convertor is a half-bridge, transformer isolated, inverter. However, such output invertors often have high switching losses and/or require passive snubber circuits (which increases losses) because each snubber must operate in both directions overall, but only in one direction at a time. Also, known snubber circuits generally have a limited range of acceptable loads and will not snub proportional to the load, thus the losses are relatively high for lower loads.
Accordingly, a power circuit that provides little switching losses and a high (close to unity) power factor is desirable. Also, the pre-regulator should be able to receive a wide range of input voltages without requiring re-linking. A desirable output convertor will include a full wave, transformer isolated, inverter, that is soft switch and has full range, full wave, low loss snubber.
According to a first aspect of the invention a welding power supply includes an input rectifier that receives sinusoidal or alternating line voltage and provides a rectified sinusoidal voltage. A pre-regulator receives the rectified input-and provides a dc bus. An invertor connected across the bus provides a welding output. The pre-regulator is an SVT (slow voltage transition) and an SCT (slow current transition) switched invertor.
In one embodiment the pre-regulator includes a snubber circuit having a diode that is SVT switched.
In another embodiment the inverter is a boost converter with a switch. The pre-regulator includes a snubber circuit having a capacitor and an inductor. The capacitor is connected to slow the switch voltage rise while the switch is turning off, and the inductor is connected to slow the switch current rise when the switch is turning on. The boost converter includes a boost inductor, a switch, and an output capacitor in another embodiment. Also, the snubber includes a snubber capacitor, a snubber inductor, a first snubber diode, a second snubber diode, a third snubber diode, a fourth snubber diode, and first and second snubber capacitors. The snubber inductor, switch, and fourth diode are connected such that current may flow from the boost inductor to any of the snubber inductor, switch, and fourth diode. Current flowing through the fourth diode can flow through either the third diode or the second capacitor. Current flowing from the boost inductor through the snubber inductor can flow through either the first diode or the first capacitor. The fourth diode and the second capacitor are connected across the switch and current flowing through the third diode can flow through either the first capacitor and the snubber inductor or through the second diode. Current flowing through the fist and second diodes flows to the output. A fifth diode is connected in anti-parallel to the switch in one embodiment.
A second aspect of the invention is a method of providing welding power by rectifying a sinusoidal or alternating input line voltage and pre-regulating the sinusoidal input line voltage to provide a dc bus. The method further includes SVT and SCT switching a boost convertor. The bus is converted to a welding output.
Pre-regulating includes, in one embodiment, maintaining a boost converter switch off, and allowing current to flow through a boost inductor, a snubber inductor, and a first diode, to the dc bus, and turning the switch on and diverting current from the snubber inductor to the switch. Current is reversed in the snubber inductor and a second capacitor is discharged through a first capacitor, a third diode, and the snubber inductor, thereby transferring energy from the second capacitor to the snubber inductor. Current is diverted through a fourth diode, the third diode and the first capacitor when the second capacitor is discharged, thereby transferring energy from the snubber inductor to the first capacitor. The switch is turned off and current diverted through the fourth diode and into the second capacitor. Voltage on the second capacitor is allowed to rise until current begins to flow from the snubber inductor to the first capacitor and then current is diverted from the second capacitor through a third diode to the second diode. The current from the boost inductor to the snubber inductor increases until all of the current from the boost inductor flows into the snubber inductor. Then current is diverted from the first capacitor to the first diode. This process is repeated.
One embodiment includes SVT turning off a diode in a snubber circuit. Another includes slowing the switch voltage with a capacitor rise while the switch is turning off, and slowing the switch current rise with an inductor while the switch is turning on to SVT and SCT switching a boost convertor.
A third aspect of the invention is a welding power supply having an input rectifier that provides a rectified voltage. A pre-regulator receives as an input the rectified signal and provides a dc bus. An invertor converts the bus to a welding output and the pre-regulator includes a power factor correction circuit.
Yet another aspect of the invention is a welding power supply having an input rectifier and a preregulator and an invertor. The pre-regulator includes a snubber circuit having a first switch in anti-parallel with a first diode, and a second switch in anti-parallel with a second diode. The combination of the first switch and first diode are connected in series with the combination of the second switch and the second diode, and the first and second switches are connected in opposing directions.
Another aspect of the invention is a welding power supply having an inverter with first and second current paths through a transformer, each in a unique direction. The first current path includes at least a first switch with an anti-parallel first diode and the second current path through the transformer in a second direction, the second current path including at least a second switch with an anti-parallel second diode. A snubber includes a current path having a third switch with an anti-parallel third diode, a fourth switch with an anti-parallel fourth diode. The third switch and anti-parallel diode are in series with, and oppositely directed from, the fourth switch and anti-parallel diode. The snubber also has a at least one snubber capacitor.
In alternative embodiments the first and second switches are in a half-bridge configuration or full bridge configuration. Also, the snubber capacitor may be split into two capacitors.
Another aspect of the invention is a method of providing welding power by turning on a first power switch and a first snubber switch, and allowing current to flow through the first power switch, a first dc bus, a first power capacitor, and in a first direction through a transformer. Then the first power switch is turned off and current flows through the first snubber switch, a second snubber diode, a snubber capacitor, and through the transformer in the first direction, while the first power switch is turning off, to provide a slow voltage transition off. Then current flows through a second anti-parallel power diode, a second DC bus, a second power capacitor, and through the transformer in the first direction, while the first power switch is continuing to turn off, to continue providing a slow voltage transition off. The first snubber switch is also turned off. After the system is at rest a second power switch on and a second snubber switch are turned after the first power switch is off, and current flows through the second power switch, the transformer in a second direction, the second power capacitor, and the second bus. The second power switch is turned off and current flows through the second snubber switch, a first snubber diode, the transformer in the second direction, and a snubber capacitor, while the second power switch is turning off, to provide a slow voltage transition off. Then current flows through a first power diode, the transformer in the second direction, and the first power capacitor, while the second power switch is turning off, to provide a slow voltage transition off. The second snubber switch is also turned off, and the process is repeated.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.